Method of measuring the particle concentration of aerosols



sePt- 20, 1950 B. A. BERGSTEDT 2,953,013

METHOD OF MEASURING THE PARTICLE CONCENTRATION OF AEROSOLS Filed June 4,1956 z u u! m w 0 o 1 2 a z.

METHOD on MEASURING PARTICLE I *CONCENTRATION OEAEROSOLS I Bengt AllanBer gstedt, Solna, Sweden,assignor to Aktiebolaget Atomenergi,Lovholmsvagen, Sweden, a Swedish company Filed June 4, 1956, Ser. No.589,132 Claims priority, application Sweden June 8, 1955 1 Claim. (Cl.73-28) where n and n are the number of particles per unit volume of theaerosol at the entrance and exit of the precipitator, respectively, isgiven by the equation:

fi p=1-e 9 where S=surface area of the collecting electrode of theprecipitator.

v=mean velocity of the particles towards this surface.

q=flow rate of the gas through the precipitator.

In this equation S is constant for a given precipitator, and for aspecified value of the operating voltage or corona current v is alsoapproximately constant; V depends also on the size and the electricalproperties of the particles and on the pressure and temperature of thegas. Thus p becomes a function of q, and this function shows a rapidfall when q is increased. Solving the above formula for q it will beseen that the flow rate q= 10g 6 p) For industrial precipitators amaximum efiiciency, i.e. a high value of p, is wanted which means that qmust be restricted to relatively low values.

The same rule has been applied to precipitators for measurementpurposes, and it has been thought that the greater accuracy in themeasurement the higher collection efliciency p should be used and thatthe flow rate q must be comparatively low.

The invention is based upon the fact that the accuracy and sensitivityof the measurement does not depend on a high value of the efiiciency p,but are on the contrary increased when p is low, for example 20 to 25%.The inventor has found that, for moderate values of the gas flow rate q,the sensitivity of the measurement is a function of the product p-qwhich increases rapidly for small q-values and tends towards asaturation value S-v when q is increased. When the electrostaticprecipitator operates at or near saturation (for not too high flowrates) the amount of dust precipitated in the unit of time will benearly independent of the flow rate q.

According to the invention the aerosol to be measured is passed betweenthe ion emitting electrode and the collection electrode at such a highrate that the product 2,953,013 Patented Sept. 20, 19 60 2 H TIZ of thecollection efliciency and said flow rate will be rac tically independentof the flow rate.

The amount A of dust particles collected in a'given time T can bemeasured in oneway or the other and'by different means, for example byweighing or, in case of radioactive dust,-'by radioactivity detectors.'What is wanted is the amount of dust per unit volume R. This is givenby the equation R-pqT=A, or

where A, q, and T are measured and p is known for a precipitator whichhas been calibrated. The efliciency p can otherwise easily be determinedby two measurements with two precipitators in series. In the firstmeasurement one gets the amounts A =Rp qT in the first precipitator Inthe second measurement, in which the precipitators have changed theirplaces, one gets and From these equations p and p can easily becalculated as all quantities except R are known. It is not necessary forthe aerosol concentration R to be equal in the two measurements.

The mode of operation of the precipitator according to the inventiongives especially the following advantages. For a given collectionsurface area and particle velocity towards this surface the number ofparticles collected in the unit of time will be approximately maximaland will be nearly independent of the gas flow rate through theprecipitator. The distribution of the dust over the collection area willbe more uniform than when the precipitator is operated at a highcollection efiiciency.

The present method is with particular advantage carried out with aninstrument described in my copending US. application Serial No. 589,131,filed June 4, 1956, titled Electrostatic Precipitator. With such aninstrument the concentration of the radioactivity of a radioactiveaerosol was determined in the following way. The air in a room, where agreat quantity of uranium ore was stored, was tested. This air wasexpected to have a relatively high percentage of radon which wouldessentially increase the radioactivity over the normal radioactivity ofthe air.

In the apparatus the collection surface was 16.0 square centimeters andthe point shaped emission electrodes were set to a distance of 5.60millimeters from the collection electrode. The apparatus had beencalibrated at a gas flow rate of 0.575 cubic meter per minute, and wasthen found to have a precipitation efiiciency of 16.6 percent. Atworking voltage the corona current was 1.60 milliamperes for pointelectrodes, i.e. about 13 microamperes per point electrode.

Experiments have shown that when the instrument is run in this waywithin the range where the value of p-q is substantially constant, thefunctions p and pq coincide with the theoretical ones within an accuracyof 1.5 percent. Thus, for instance, a variation of the air flow ratewith 15 percent from the value given above renders a variation of only 2percent of the pq-value, that is the sensitivity.

The theoretical functions p and pq are shown in the attached drawing asfunctions of the dimensionless variable 2,953,013 v v i "-4fimhatjsifelaingeiisn,4H. "bnly aninimfiteriariircfease'inprecifiitationfdtefthe'efli- ,gflA method of measuring with greatacpuracy thepar: ciency 9 tlpre i atp fibcin 'rl s n th 2.5 .mrqsntticle concentration of an aerosol by passing the same and measming theprecipitate obtained on said collection between an ion emittingel'ebtrode and a collection e1ec-, electrode during the passage -of theaerosol.

t ed .fl zz n elec os a :PEQQ Pi aFT-I :w l --=ma. main n 5 7 began Saidalectmdfqs- Potential- 9 7 "Refetences CitedJnJhe file of this patent ruei gi olta t c u -p p t nfo t q 'f fheflaerosol -onto said -col;leetionelectrode, the aerosol 5 I 5513; ii'ate -oj 'flow; being within the-1'egion of xelatively .high 2,337,502; T ZBeinneyi fe't' #31111. 5,1943 Slow; rates whefe an essential increase in flow rate renders 102,484,202 Wintermute 1, 1944

